1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems use a network of base stations to provide wireless connectivity to one or more mobile units. In some cases, the mobile units may initiate wireless communication with one or more base stations in the network, e.g., when the user of the mobile unit would like to initiate a voice or data call. Alternatively, the network may initiate the wireless communication link with the mobile unit. For example, in conventional hierarchical wireless communications, a server transmits voice and/or data destined for a target mobile unit to a central element such as such as a Radio Network Controller (RNC). The RNC may then transmit paging messages to the target mobile unit via one or more base stations or node-Bs. The target mobile unit may establish a wireless link to one or more of the base stations in response to receiving the page from the wireless communication system. A radio resource management function within the RNC receives the voice and/or data and coordinates the radio and time resources used by the set of base stations to transmit the information to the target mobile unit. The radio resource management function can perform fine grain control to allocate and release resources for broadcast transmission over a set of base stations.
A conventional base station provides wireless connectivity within a geographical region that is referred to as a cell or a macrocell or a sector. Conventional base stations can transmit signals using a predetermined amount of available transmission power, which in some cases is approximately 35 W for a base station. The range of the macrocell is determined by numerous factors including the available transmission power, angular distribution of the available power, obstructions within the macrocell, environmental conditions, and the like. For example, the range of a macrocell can vary from as little as 300 m in a densely populated urban environment to as much as 10 km in a sparsely populated rural environment. The coverage area can also vary in time if any of these parameters changes.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as base station routers, that implement distributed communication network functionality. For example, each base station router may combine RNC and/or PDSN functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Base station routers wholly encapsulate the cellular access technology and may proxy functionality that utilizes core network element support to equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base station router proxies by through equivalent Mobile IP signaling. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the separate RNC and PDSN entities in hierarchical networks may be reduced or removed.
At least in part because of the reduced cost and complexity of deploying a base station router, base station routers may be deployed in locations that are impractical for conventional base stations. For example, a cluster of base station routers may be deployed in a commercial building to provide wireless connectivity to people working in the building. Base station routers deployed in a commercial location (and the areas served by these base station routers) are typically referred to as femtocells because they are intended to provide wireless connectivity to a much smaller area that encompasses the building or a portion of the building. Femtocells have a much smaller power output than conventional base stations that are used to provide coverage to macrocells. For example, a typical femtocell has a transmission power on the order of 10 mW. Consequently, the range of a typical femtocell is much smaller than the range of a macrocell. For example, a typical range of a femtocell is about 100 m.
Femtocells are expected to be deployed in conjunction with a macro-cellular network in an overlay configuration. For example, a macro-cellular network may be used to provide wireless connectivity to a district that includes numerous commercial buildings. Any mobile unit traveling through the district or located in one of the buildings can access the wireless communication system using the macro-cellular network. Clusters of femtocells can also be deployed in one or more of the buildings to provide overlay coverage within (or near) the building. Consequently, there will be a one-to-many relationship between the macrocells and the femtocells within the coverage area of the femtocell cluster. However, user equipment will typically only be able to camp on the femtocells in the cluster when the user is an employee of the company that installed the femtocell cluster or other authorized person.
User equipment or mobile units therefore need to verify that they are authorized to access femtocells in the cluster before handing off from the macro-cellular network. For example, when a user arrives at work their mobile unit may be able to detect a macrocell and numerous femtocells in clusters associated with different companies in the same building or nearby buildings. However, the mobile unit is only authorized to camp on femtocells in one cluster and make circuit-switched calls or initiate packet-switched sessions via the femtocells in the cluster. The conventional practice is to drop any active calls and then determine the correct femtocell while the mobile unit is in idle mode, e.g. by trial and error. For example, user equipment may attempt to camp on each available femtocell until it detects a cluster femtocell that allows it to camp. Since the user equipment may not have any information that can guide the selection of candidate femtocells in the correct clusters, this brute force technique can consume significant overhead and degrade the user's quality of experience. Another conventional practice is to utilize trial and error methodology to identify the handover target femtocell. For example, the RNC can try to handover the session to one femtocell in the cluster and when the handover attempt is rejected, it re-tries with another femtocell until the femtocell that accepts this user equipment is determined. Since there is no association between the scrambling codes of the femtocells in the cluster and the user equipment in the conventional approach, the RNC does not know where the user equipment should move to and therefore may attempt many macrocell to femtocell handovers to attempt to find a femtocell in the correct cluster. This not only creates a huge signaling overhead on the radio and infrastructure that increases the interference level and reduces the network capacity, but also degrades end user's quality of experience.